Rotor slot insulation systems as found in conventional large dynamoelectric machines employing forged rotors of magnetic material which include machined slots are conventionally cooled by the flow of coolant gas in subslots through openings in conductors and insulating layers. The manner in which the rotor slots are shaped and insulated as well as the efficiency of the coolant flow channels for dissipating heat from the windings present formidable space utilization design problems particularly where high maximum permissible current limits are contemplated. For example, the copper bars in a rotor slot are insulated from the magnetic material of the rotor through the use of slot armors or sheets of insulation. Since the rotors of large dynamoelectric machines include subslots for carrying a coolant gas to the radial flow channels in the copper windings, the windings in addition to being insulated from the rotor material must be supported above the subslots. It is conventional to perform such functions by laying a relatively thick piece of insulation known as a subslot cover at the junction of the rotor slot and subslot where in addition to performing the support and insulation functions, the subslot cover includes openings in the insulation to allow the coolant gas to pass from the subslot to the copper. Such openings, of course, reduce the electrical path length from the conductors to the rotor and, therefore, provide an electrical creepage path between the windings and the bottom of the rotor slot. Added to the noted insulation problem is the fact that the gas coolant itself may be contaminated with electrically conductive particles such as carbon dust.
In light of such problems subslot covers of the prior art have taken quite complicated shapes, such as that which is illustrated in FIG. 1, for example. FIG. 1 illustrates a subslot 10 and a full length subslot cover 1 having the illustrated cross section and which includes radially directed ventilation passages 6 along its length that have been machined on a two to four inch pitch. The short legs of the L-shaped slot armors, 2 and 3, are inserted into narrow slits on each side of the subslot cover and are bonded to it prior to assembly into the rotor slot through the use of an adhesive.
Such a subslot cover is illustrated in FIG. 1 of commonly assigned U.S. Pat. No. 4,859,891 issued to Jenkins et al on Aug. 22, 1989. This commonly assigned patent additionally illustrates several embodiments of another design approach wherein the subslot would be machined to the same width as the rotor slot. Such an approach permits the insertion of a U-shaped slot armor, for example, which extends throughout the slot and subslot wherein the winding conductors are supported above the subslot through the use of a second U-shaped insert that snugly fits inside the U-shaped armor in the bottom of the slot. Although such an approach eliminates the need for a subslot cover, it could be improved from the standpoint of space utilization.
Another approach would include two straight sheets of armor with a thick creepage block at both the top and bottom ends of the slot. Although the subslot cover of such an embodiment would be less complicated than that illustrated in FIG. 1, it provides poorer radial space utilization which results in the use of larger rotors as well as requiring additional machining so as to provide ventilation openings in the creepage blocks.
A still further approach would involve the molding of a large number of short plastic pieces which when snapped together form a subslot cover having the cross section shown in FIG. 1. The axial length of such pieces would coincide with the ventilation opening pitch resulting in each piece having one ventilation opening. The pieces would be assembled together with a single U-shaped armor by pressing one of the pieces through an opening cut in the bottom of the U-shaped armor with a mating piece snapped onto it from the other side of the armor. Such assembly would be repeated for each of the openings over the length of the armor. Clearly the latter approach is complicated and time consuming.
I have discovered that in dynamoelectric machines utilizing rotors with subslots, a rotor slot can be insulated using two slot armors each with an extra fold which thereby extends the armor part way into the subslot thus making use of both sides of the armor surface and extending the creepage path almost entirely in the radial axial plane. Such an extended creepage path provides surfaces which tend to be self cleaning since they do not collect carbon dust and other contaminants from the gas coolant. Additionally, my new concept for rotor slot insulation would eliminate the need for a subslot cover.
Accordingly, the primary objective of my invention is that of providing a simpler, more versatile rotor slot insulation system with a minimum number of parts but which provides more uniform and efficient cooling of the rotor.
These and further objects and advantages of the present invention will become more apparent upon reference to the following specification, appended claims and drawings.